The theory of magnetic anomalies provide solid proof written in the ocean floor to show that new crust was being created here. The earth's magnet field changes from time to time over periods of thousands of years. This change is record in rock.
The new rock formed along the sea floor is an accurate record of the changes in the magnetic field. Seafloor spreading occurs at divergent margins where two plates are being pulled apart. As this happens, magma upwells and new crust is formed.